The present invention concerns a new method of adding an alkaline earth metal to a large-pore zeolite. This method completely eliminates the acid function from the zeolite and inserts the alkaline earth metal into as many exchange sites as possible. The resulting catalyst has a high selectivity for dehydrocyclization.
Catalytic reforming is well known in the petroleum industry. It involves treating naphtha fractions to improve the octane rating by producing aromatics. The hydrocarbon reactions occurring during reforming operation include dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, dehydrocyclization of acyclic hydrocarbons to aromatics, dealkylation of alkylbenzenes, isomerization of paraffins, and hydrocracking reactions which produce light gaseous hydrocarbons, e.g., methane, ethane, propane and butanes. Hydrocracking reactions should be minimized during reforming as they decrease both the yield of products in the gasoline boiling range and the yield of the hydrogen.
Because of the demand for high octane gasoline for use in motor fuels, extensive research is being devoted to developing improved reforming catalysts and catalytic reforming processes. Catalysts for reforming processes must be able to produce high yields of liquid products in the gasoline boiling range and low yields of light gaseous hydrocarbons. The catalysts should possess good activity in order that low temperatures can produce a quality product. The catalysts should also either possess good stability, in order that the activity and selectivity characteristics can be retained during prolonged periods of operation, or be sufficiently regenerable to allow frequent regeneration without loss of performance.
Catalysts comprising platinum, for example, platinum and rhenium supported on chlorided alumina, are widely used for the reforming of naphthas.
In conventional reforming, the hydrocarbons to be converted are passed over the catalyst, in the presence of hydrogen, at temperatures of about 450.degree. C. to 550.degree. C. and pressures of about 50 to 500 psig. Part of the hydrocarbons are converted into aromatic hydrocarbons, and the reaction is accompanied by isomerization and cracking reactions which also convert the paraffins into isoparaffins and lighter hydrocarbons.
The conventional catalysts have given fairly satisfactory results with heavy paraffins, but less satisfactory results with C.sub.6 -C.sub.8 paraffins, particularly C.sub.6 paraffins. Catalysts based on a type L zeolite are more selective with regard to the dehydrocyclization reaction and produce excellent results with C.sub.6 -C.sub.8 paraffins.
Still more selective for dehydrocyclization of C.sub.6 -C.sub.8 paraffins are large-pore zeolitic catalysts that contain an alkaline earth metal and at least one Group VIII metal. Such zeolitic catalysts are disclosed in U.S. patent application Ser. No. 344,572 now U.S. Pat. No. 4,435,283.